RING the bells! A Remanian revolution has broken out in the UK Labour Party. The Shadow Chancellor, John McDonnell, has announced that he will campaign for Britain to stay in the European Union, if and when there is a repeat referendum on Brexit. The Shadow Home Secretary, Diane Abbott, leapt up to endorse his “brilliant” pivot to the anti-Brexit camp.

Europhiles in the Labour Party can hardly believe it. After three years trying to explain to bemused voters why Labour is backing Brexit (even though it, er, isn’t) they’re now free to say what they always wanted: Labour is the party of Remain. It’s out of the closet.

Twitter trolls have been sent packing. The high priestess of Remain, the Guardian’s Polly Toynbee, has given her blessing. The hyperactive Labour peer, Lord Adonis, and the #FBPE philosopher, SC Grayling, are singing a duet of “Oh! Jeremy Corbyn”. Tony Blair has been stunned into silence.

However, long-standing Labour Remainers are just a little cautious about this conversion on the road to departure. And with cause. Mr Corbyn remains enigmatic, as always. It is not at all clear the Labour leader is personally planning to back Remain.

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Moreover, Labour Party policy is still pro-Brexit. Its manifesto commits a Labour government to accepting the results of the 2016 referendum and leaving the European Union, ending free movement along the way. A General Election is possibly weeks away, yet an incoming Labour government is committed to negotiating a “better” Brexit deal with Brussels.

The new twist is that this new deal would be put to the voters of Britain in a “confirmatory referendum”. But there is no presumption that Labour would campaign in favour of it. Indeed, leading figures like Ms Abbott and Mr McDonnell would clearly call for voters to reject Labour’s new deal.

Let’s work this through: Prime Minister Corbyn’s first act would be to approach Brussels for renegotiation of the Withdrawal Agreement. This would presumably include the things they wanted to tag on to Theresa May’s deal: Customs Union, regulatory alignment with the single market, job and environmental protections and so on.

Now, there is every reason to suppose that Brussels would accept this new deal with alacrity, since it is very largely the arrangement it has with countries like Norway and Switzerland. Michel Barnier, or his successor, would welcome a prodigal Britain back into the fold. There would no longer be an Irish backstop because there would be no hard border. Job done.

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However, we’re told that Labour would then put this to the people in a referendum in which most of the Labour Cabinet would vote to reject the deal it has just negotiated. Labour has always faced both ways on Brexit, but the new policy has turned incoherence into a fine art.

How would this policy be framed in Labour’s General Election manifesto? We will honour the 2016 referendum and negotiate a jobs Brexit, which we will advise the people of Britain to reject. And we’re not giving the referendum question because that would be “speculative”.

Some in the Labour Party still believe that the “confirmatory ballot” could be Deal versus Remain, excluding any No Deal Brexit. But this would surely be a non-starter. The Electoral Commission would say that such gerrymandering of the franchise excludes too many voters. According to YouGov, more voters are in favour of No Deal Brexit than are in favour of Mr Corbyn becoming Prime Minister.

The last time Labour got into a mess like this was over the Scottish referendum back in 1996. Tony Blair did a U-turn on devolution announcing that, if the Scottish Parliament was really the “settled will” of the Scottish people, as the late John Smith believed, then it should be tested in a referendum. The wrangling went on for months. The hapless Shadow Scottish Secretary, George Robertson, was left trying to explain why there would be one, two or possibly three referendums before the settled will could be determined.

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However, that constitutional imbroglio was nothing compared to Brexit. Scottish home rule was never going to determine the outcome of the 1997 General Election because it was of marginal interest to the vast majority of UK voters. Brexit is the central question that will dominate the coming General Election, and probably the ones after it. Parliament has proved unable to address the question properly, let alone find a satisfactory resolution.

If Labour had really intended to pivot to Remain it should have done it two years ago when Parliament debated Article 50. That legislation made Brexit legally binding, and a No Deal Brexit the default. It was the height of irresponsibility for MPs, led by Mr Corbyn, to vote by a massive 384 majority to authorise Brexit, on a hard timetable, irrespective of the terms of departure. It was, as the SNP argued, a Brexit blank cheque.

Mr Corbyn now says he will do “whatever it takes” to stop No Deal Brexit. But this is like waiting till the bus is tipping over the cliff before trying to do a handbrake turn. Labour could belatedly have prevented a deal-less departure from the EU by supporting Mrs May’s Withdrawal Agreement, most of which Labour actually agreed with. It chose instead to walk through the division lobbies with the far-right European Research Group of Tory MPs.

Indeed, Labour could have halted hard Brexit on a number of occasions. It refused to endorse a Norway option, Mrs May’s deal, or a compromise in the last-minute negotiations with the Government. Mr Corbyn chose to dance around the question, seeking a narrow party advantage. It was all tactics and no strategy and played into the hands of one Boris Johnson.

My own view is that Mr Corbyn would be rather content now to see a hard Brexit. This is partly because he has always been a Eurosceptic, but mainly because he thinks Labour could capitalise on the chaos thereafter. When your opponents are making a mess of things, don’t get in the way.

It’s an electoral gambit, but a cynical one and voters may not go along with it. When the history is written of how the UK came to this situation, driven by a band of hard-right Brexiters over the cliff, Mr Corbyn may find he is cast as Mr Johnson’s little helper.